Collection and removal of mine gas



RN Qfim 4mm May 23, 1950 CGLLECTION AND REMOVAL OF MINE GAS E a M 3% a F.3

INVENTOR. R NK A. HOWARD BY w- @N om m m j P wm \Qw M & Q E

hm mm CNN 5 {g a Patented May 23, 1950 -UNITED STATES PATENT "OFFICE I fI 2,508,949 I i r 1. COLLECTION AND REMOVAL or MINE GAS E Frank A.Howard, New York, N. Y.

Application April 7, 1949, Serial No. 86,081

s'claims. (01. 262-34) contain'50% or more ofthe original coal, and gasin substantial amounts may be slowly re-.

leased from this coal and from the marginal Walls of the mined-outsection for years or even decades. Because of the volume and persistenceof the flow, the gas has been sometimes assumed to come into the minefrom lower gas reservoirs, but I have found that it is unnecessary toassume communication with any extraneous source of gas because the totalvolume, rate of delivery, and rate of decline of the mine gas are allconsistent with experimental determinationof the absorbedgas contentofcoal samples.

To release this gas and prevent the building up of a gas pressure in thesealed chambers, it is customary to provide large open vent pipesextending from the sealed chambers to the surface. Although these ventpipes arelarge enough to discharge gas to the atmosphere at ratesgreater than the rate of-evolution of gas within the sealed chambers,it-is nevertheless found that pressures appreciably above or belowatmospheric occur within the chambers quite regularly. This is due tothe changes which occur, sometimes with great rapidity, in thebarometric pressure.

For example, a change of barometric pressure of one inch of mercury mayoccur within a period often hours or, in the event of a violent windstorm, in a much shorter period. In a typical mine two sealed-offchambers contain respec tively- GELOOQJJOO and 120,000,00 cubic feet ofgas, these being the volumes of coal which are mined out of these areasbefore they were sealed off. In the smaller chamber, such a change ofbarometer would .require. admission of gas to the chamber or escapeofgas from the chamber at a rate of about 200,090 cubic feet per hour ormore. This quantity of gas cannot be delivered through along vent-pipeof normal size, for example an 8'. pipe, without a substantial pressuredifference.

.It follows. therefore, thatln. practice sealed mine chambers mustbreathe through their vent water or more, during a rapid fall of thebarom eter. In consequence there is usually some pipes at very.substantial rates as the barometer changes, and this breathing action issuperimposed upon the intended function of venting the gas which isslowly evolved from the underground coal faces. I

There are two unfortunate consequences of this breathing action. .In thefirst instance, the

pressure in the sealed chamber sometimes builds,

up to a substantial figure, for example 6" of leakage of gas through oraround the seal and into the active area of the mine. This hazard makesit desirable toinstall a special split in the mine ventilation system,by which an entirely separate current of ventilating .air is passedcontinuously over the mine face of the seal to carry away such leakinggas without permitting it to enter the main ventilating cur rent. Asecond hazard arises when the barometric pressure increases rapidly.Underthese conditions the vent pipes inhale air instead of venting gas,and air also tends to leak into the sealed chambers through or aroundthe seal. Thus there may be created a zone of very large volume in thesealed underground chamber in which an explosive mixture of gas and airmay be found. The accidental ignition of this mixture from naturalelectrical discharges, chemical ac tion, sparks from roof falls, istherefore a possibility.- o

1 Safety in undergr'oundmining operations requires that, to the fullestextent practicable, explosion hazards should be eliminated by preventingthe formation of explosive mixtures as well as by preventing ignition ofsuch mixtures. It is the purpose of my invention to improve theconstruction and operation of the equipment used in underground mines ofthe type described so as to minimize or eliminate the existing hazardsand at the same timeto recover and de liver the gas which is slowlyevolved from the coal in the purest practical form. My invention will befully understood from the following specification taken in connectionwith the annexed drawing in which Figure 1 illustrates in diagrammaticsection a mine 111+ stallation in which the invention is utilized andFigure 2 shows in fragmentary section the de-' tails of the ventvalve. wf In this drawing the numeral 1 designates a coal pillar or rib in thecenter of'an active entry 2 in an underground coal mine. The entry 2 andthe active workings with which it connects are ventilated in the usualway by fresh air, at least part of which passes through the entry 2. Inthe plane of the drawing the entry 2 connects with cross entries 3 and 4which lead to lateral areas of the mine which have been worked out tothe existing economic point. It it well known, however, that as miningmethods improve and prices advance it is often desirable to re-open suchmined-out areas and extract further quantities. oi coal. Forthis reasonit is desirable to avoid any action which would preclude thispossibility or make it unduly expensive to re-- open the mined-outareas. The entry 3 is cut off from the mined-out section-by aninner seal5. The seal 5 is of masonry, recessedinto the walls of the entry asindicated to reduce leakage around the seal. While only one is shown, itwill be understood that main entries usually require several such sealssince they may consistof multiple passages separated by ribs or pillars.Spaced away from the inner seal 5, by a distance of perhaps 100 feet, Iprovide' an outer seal ,6 which may be of. identical constructionor maybeless costly for-the reasons which will hereafter appear.

The chamber 1 beyond the inner seal may be of great size,ifor example60,000,000 cubic feet-free space, representing the volume of the coalwhich has been removed. There remains in this chamber an equalvolumaymore or less, of coal which it :was not economical to mine outunder the existing conditions. From this coal combustible gas,consisting essentially of methane, is con-' tinuously released at a ratewhich may be, for example, 400,000 cubic feet per day for many years.after the chamber 1 has been sealed off and all active miningoperations therein discon- To permit the gas to escape I provide a ventpipe 8, which may be of about 8" diameter and which passes through theinner and outer seals, extends into the mouth of the cross entry 3 andis there equipped with a hand-operated shut-off valve 9'. Beyond. theshut-off valve the vent pipe 8 turns upwardly, passes through the coverover the coal, which may be many hundreds of feet in thickness, andemerges at the surface of. the ground where it is extended to a,convenient height and equipped with a non-return safety type of ventvalve f which will be later described. The cross entry 3' is cut ofifrom the active entry '2 by a timber partition I l which permits aseparate stream of ventilating air to be circulated over the outer faceof, the outer seal 6 without admixture with. the .main current of airi-ntheactive entry '2. The entry 3 "is accessible from the active entry '2for inspection purposes at any time by .a suitable door, not shown, inthe timber partition I l.

The chamber designated GL between the inner seal and outer seal 6 isutilized, in accordance with my invention, as agas-lock which is keptcontinuously filled with gas withdrawn from the chamber 1 and at apressure always equal to the pressure in the active area cf the mine.This result is accomplished by three novel procedures.

First, the gas from chamber 1 is continuously and forcibly withdrawnfrom this chamber, through the vent pipe 8, by :a blower or'low pressurepump 12 located at the surface and taking suction on the .vent pipe 8through a suction line l3. The-gas discharged from the blower l2 may bedelivered directly to some point of consumption, 'or as will laterappear it may be delivered to another sealed chamber :in the same mineby a transfer line or delivery line "H.

The gas-lock between the seals 5 and B is connected with the surface bya relatively small line I5. At the surface this line is branched, onebranch connecting with the blower suction line l3 and the other branchwith the blower delivery line H. Each branch is equipped with aregulating valve, as indicated diagrammatically at 18. The two valves I6are operated by a pressure regulator shown diagrammatically at I! in theform of a chamber dividedby' a flexible diaphragm to which connectinglinks from the two valves iii are afiixed. The pressure on one side ofthe diaphragm is that of the gas-lock, while the pressure on the otherside is that of the entry 3 whichis the same as the pressure in theactive mine adjacent the gas-lock. These two underground pressures mayconveniently be transferred to the regulator H at the surface by meansof small tubes l8 and I9 led down through the vent pipe 8 andterminating respectively in the gaslock and in the entry'3. I prefertomake a small opening 20min the vent pipe 8 within the gaslock so as toensure a constant small flow of gas through the gas-lock.

The construction of the vent valve is shown diagrammatically by thedetailed section at the top of a second vent line 20 and by Figure 2.The essential operating features of the valve are that it should permitfree escape of gas under low back-pressure, that it should prevent allingress of air under any condition, that it should not chatter orvibrate, that is should not stick, freeze tight or corrode, and'thatit-shouldprevent flashback of a flame or explosion through the vent pipeif a corribustiblemixture should at any time exist.

These operating essentials are met by a valve comprising a horizontalpallet 21 of very light weight which seats by gravity on a horizontalseat, is loosely guided by vertical spaced bars 22, which support thehood over the valve, and restrained from tipping or chattering by aloose link connection with the piston of a damping cylinder 23 filledwith light oil. The valve is protected by a hood 24 and flash-back offlame is prevented by a safety screen 25 surrounding the vertical hoodsupports which form the valve cage. The entire valve construction is ofrust-proof metals.

The second vent'pipe 20 communicates with a second sealed chamber 26 ofvery large capacity, for example 120,000,000 cubic feet. The chamber 26is shown as cut off from the active mine by a single masonry seal 21.The entry 4 in which this seal is erected is separated from the mainentry 2 by a timber partition 29 which permits entry 6 to beindependently ventilated. The arrangements here shown are the same asfor the chamber I save for the omission 'of the outer seal forming thegas-lock provided for chamber 1.

The discharge line H! from the blower I2 is led into the chamber 26 by aconnection 28 which extends directly downward from the surface throughthe cover above the coal.

At a point removed by a great distance, for example one mile, from theseal 21, vent 20 and transfer connection 28, there is provided a gasdelivery pipe 3!]. This delivery pipe 30 may also be equipped with anautomatic vent valve as heretofore described so that it may exhale gasdirectly to the atmosphere if necessary to relieve pressure in thechamber 26, but may not inhale air. The pipe 30 is in fact a gas welland serves as the outlet for the entire gase production oi. the chambersI and 26. For this purpose it car- 5 ries anoutlet connection 3| whichdeliversinto the su'ction side of a pump 32. By means or this pump the'gasmay be delivered'under'any desired pressure to a fuel gasdistribution system or to any consuming outlet.

It is important to regulate the quantity of gas withdrawnfrom chamber 26by the pump 32 so as to "obtainthe maximum gas recovery whileredeclines. For such regulation it is necessary at all 'tim'esto know boththeabsolut'e' pressure in the chamber 26 and the barometric pressure. Toobserve the absolute pressure in the chamber 26,

without any'possible error due to velocity eifects and pressure drop inpipe 30, I introduce a sep-" arate pressure tubing 33 through a fittingon the pipe above the surface, and connect thispr'essure tube witha'gauge 34 which indicates, or if desired also records, the absolutepressure in chamber 33 at a point far enough from the opening of thepipe 30 to avoid velocity effects. In close proximity to the pressuregauge 33 I provide a barometer 35, which may also be of therecordingtype if so desired.

In theoperatiOn of the installations described above the sealed chambers1 and 26 areat all times free to discharge gas to the atmosphere throughthe relief valves ID on the ends of the vent pipes 8, 20 and 3D.These'valves may be designed to open under a pressure less than one inchof waterjandso offer a minimum of resistance to the discharge of gas atperiods when the barometer falls below the pressure in the chambers. If1 it is desired to conserve the gas and if tests show that there is verysmall leakage of gas from the sealed chambers into the active section ofthe mine, whenthe pressure in the chambers is higher than in the mine,the valves may be loaded ,to"

remain closed until the barometer drops as much as one half to one inchor more of mercury below the pressure in the chambers?" Such" setting"hr the valves will prevent'wast-e or gas through the" veritsunder allcommon fluctuations'of the barometerfbut still leave' them effective forstorm conditions. It is common to encounter water in coal mines and inabandoned and sealed sections roof falls and squeezes may produce watertraps through which the gas must pass on its way to the vents. Thesewater traps have been found to cause pulsations in gas delivery whichresult in noisy and destructive chattering of the valves. For thisreason all valves are preferably fitted with dampening devices such asthe hydraulic cylinders illustrated. Despite all precautions it is alsopossible that a combustible mixture might sometime be formed in thechambers'andvent pipes; Such a mixture might be exploded throughatmospheric electrical dischargesand it is, therefore, also verydesirable to equip these relief valves with metal safety screens asshown at25 so that no flame can be propagated through an opeii'valve.The valve itself in the form illus-' trated however, a very effectiveflame arrester because the velocity of efflux of the gases through 1 theopening between the valve and itsseat'is always very substantial,corresponding to the loading'of the valve, 'andwili usually exceed therate of flame propagation.

From'the foregoing it will be understood that v as a basic principle ofmy invention the sealed chambers remain indirect connection with theconnection to the blower inlet 1 3.

atmosphere through large diameter vent pipes which are alwaysefiectiveto prevent the build up of any dangerous gas pressure in the chamber s,but that through the installation of suitable vent valves and safetyscreens on these pipestheentry of air into the chambers is pos-" itivelyprevented, flash-back of flame through the vents is prevented, and byloading the valves Within limits approximating the most commonbarometric changes, loss of gas to the atmosevolved from the coal in thesealed chambers,

there is provided the blower l2 and pump 32, and although the detailedarrangements used for that purpose are of course subject to change asrequired by variations in local conditions, the principles are wellillustrated by the installations shown and described. Thus it isdesirable, where maximum rate of recovery of gas is wanted, to carry amean pressure in the sealed chamber below the mean pressure of theactive mine. This lowered pressure accelerates the rate of release ofgas from the coal. Where it is necessary to maintain the highestpractical heating value on the gas, in-leakage of even small proportionsof air through the seal or through by circulating gaswithdrawn fromchamber I The circulation is controlledby the pressure regulator l!which is, through by the blower 12.

the pressure tubeslB and i9, responsive to the pressure 'diiferentialbetween the gas-lock ai-id' the active mine." -Should"the pressure inthe gas-lock tend to become lower than that in the mine the regulatoropens the valve in the connection from the line i 5 to the blower outletl4, and simultaneously closes the valve in the delivered to thegas-lock, building up the pressure until it becomes equal zed.Correspondingly, pressure in the gas-locl above that of the active minepassages 2 and 3 acts to open the line I 5 to the suction side of theblower and quickly restore the equilibrium. In all cases the hole 29a inthe vent line within the gas-lock ensures a circulation of gas throughthe lock and prevents it from becoming a dead-end, which by slowin-leakage or diffusion of air might become an explosion hazard. Ihehole 280. may

be considerably smaller than the tube l5 so that it cannot prevent themaintenance of the desired. pressure equilibrium between the gaslock andthe active mine. In. setting the pressure regulator i1 allowance must bemade for the difference in specific gravity between the gas and air.Coal mine gas of good quality and free from substantial contaminationwith air is in fact "natural gas. recovered from non-carboniferousformations it Gas is thus Like the natural gas is principally methaneand has ;a specific gravity in the approximate range of 57 to .64 andwith deep mines the resultant difference in the weight of the column ofgas in the pipe l8 and the "column of air in the pipe l9 may be over 6"of Water. This correction if not made and maintained will render thegas-lock ineffective as a practical means for closel balancing gas andair pressures and avoiding contamination of the mine air by out-leakageof gas on the one hand, or danger of creating an explosive mixturebehind the seal byin-leakage of air on the other hand. In order tomaintain constant conditions corresponding to the correction made asabove, it is desirable to provide a small air hole in the tube 9 at thesurface to ensure very slow circulation of air through this tube, andalso to provide a very small gas connection between tube l8 and theinlet line is of the blower, as indicated at Ma. In both cases theamount of circulation is too small to create velocity effects or give anerror in the pressure reading but sufficient to ensure constant qualityof gas in tube I8 and air in tube 59 corresponding to the calculatedcorrection for the difference in weight of the gas and air columns inthe two tubes.

The blower l 2 is driven by some suitable primemover, not shown, toapply suction to the vent 8 and forcibly remove the gas and deliver itunder pressure into the outlet line 14. It is regulated to operate at aconstant rate of delivery .less than or equal to the rate of evolutionof gas from the coal in chamber I, and as indicated above, thisevolution rate is itself affected by the absolute pressure in thechamber. By the use of the gas-lock GL in which the pressure isequalized with the mine pressure the chamber 1 may be carried atconstant absolute pressure below the mean pressure in the mine. Gasrecovery is thereby accelerated, and at the same time mine safetyconditions are improved. There is no out-leakage of gas through oraround the outer seal 6 because of the equilibrium of pressures, andwhile there may be appreciable leakage from the gas-lock; into thechamber 1, it is .gas only which enters the chamthe .oasl'e of mostlarge mines a number of enamels an :bacr'eated successivelyas ngeperatioris proceed. Insuch cases it is desirable to utilize one or thesecham bers as a delivery chamber for all gas to be furnished toconsumers. I have shown the chamber 26 as used for this purpose. The gasfrom the blower outlet !4 is, therefore, discharged through a directconnection 23 into the chamher I. This leaves the full capacity of ventpipe available as an outlet for emergency relief of excessive pressuresin chamber 26. The chamber chosen as the delivery chamber willpreferably be one of very large volume. This makes it most eifective asa pressure equalizing and mixing reservoir and has the additionaladvantage of permitting the outlet to belocated at a point so farremoved from the inlet and from the seal and vent 25* that any air whichmay enter by leakage at these points will remain for long periods incontact with the coal in the chamber 26 before reaching the outlet pipe30. I have found that under these conditi-ons the oxygen content of theleakage air disappears by slow'chemical reaction with the coal to "formcarbon dioxide. This is very desirable Where the gas is to be deliveredinto a distribution system.

As a-specific example :of the method of operation of the equipment andinstallationabove described the following is cited. The sealed chambersare filled with air from the mine ventilation system up to the time whenthey are cut off from the active mine, and it follows that there in along introductor period during which the gas slowly evolves from thecoal replaces and expels the air. Throughout this period large Zones ofexplosive mixture necessarily exists in the sealed chambers and it istherefore very important that the valves and safety screens functionperfectly to avoid any flash-back. In the course of time this hazarddiminishesas the gas percentage increases and the-oxygen content of theresidual air is reduced by slow-combination with the coal to form CO2.When the'pumps are started the valve 10 on vent pipe 8 is loaded to openat a pressure of one inch of water. The mean barometric pressure at thesurface of the ground in thelocality in question is 29 inches ofmercury, and correcting for the height of the air column in the mineshaft and the pressure loss of the mine ventilation system the meanpressure in the active mine is 29.5 inches of mercury. The blower I2 isregulated to maintain an absolute pressure of 28.25 inches of mercury inthe-chamber "I, under which condition it is found that the blowerdelivers 400,000 cubic feet perdayofgas having a specific gravity of ,60into the transfer line 14. This is the rate of evolution of the gas fromthe coal in chamber 1 and will be found to be substantially constant forany constant absolute pressure, declining slowly with the passage oftime. The blower I 2 may deliver the gas at a pressure of 5 pounds persquare inch, which is adequate to discharge the gas into the chamber 25through line 28. Through the valve l5 gas enters the pressure equalizingline l5 and is conveyed to the gas-lock, building up the pressure thereto the same level as in the entry 3. This is a mean pressure of 29.5inches but changes continuously with the fluctuati ens of the barometerand on some occasions in I e the absolute pressure of dhamber I. Whenthis ll opens the line .sbctren'line of the blower until the equilibriumis reached: iie I3 is smallenough so that thereisalways-sa gpr'essurdrop-l "the line s'ufiieient to operate the system astlesfl If thebarometric pressure at the surface falls sufiiciently or if the bloweri2 ceases to operate for any-"reason, the automatic valve on the vent Itopens to release gas directly from chamber 1 to the atmosphere and thusprotect the active mine against in-leakage of gas from the chamber.

Under normal operating conditions, the gas delivered into chamber 26through transfer line 28, together with the gas evolved from the coal inthis chamben-is continuously removed by the well-pipe 30 and pump 32,and may be delivered at high pressure directly into a long-distancetransmission line. The pump 32 may be regulated to hold, for example, inchamber 26 an absolute pressure of 29.5 inches of mercury correspondingto the meanrpressure in the active mine. The relief valves on the vent20 and well-pipe 30 may be loaded to open at a pressure of 3 inches ofWater, above atmospheric pressure, it having been determined that thismuch pressure differential may .be .tolerated without important leakageof gas from the chamber 25 into the active mine. Since the mean pressurein th chamber 26 is slightly above the mean pressure of the mine thereis no danger of in-leakage of air into chamber 26 save during brief andinfrequent periods of unusually high barometric pressure, and the largeflow of gas into the chamber 26 adjacent the seal 21 through thetransfer line 26 prevents this in-leakage from forming a zone ofexplosive mixture near the seal which might otherwise occur. As has beenpointed out, occasional small in-leakage of air at this point does notappreciably affect the quality of the gas delivered through vent 30because of the remote location of the latter and because in its travelthrough the maze of passages which exist in the worked-out area theoxygen content of the air disappears and is replaced by a much smalleramount, one-half or less, of carbon dioxide. In the event the deliverypump 32 should cease to operate the vent valve on top of well-pipe 30 isavailable to re lease gas from the chamber 26, supplementing the valvein vent pipe 20.

I am aware that it has heretofore been proposed to recover gas fromvirgin coal seams in situ by specially piercing such seams with bores,drifts, and tunnels and applying .a vacuum.

The present invention is not directed to this problem but to the specialproblems which arise when active mining operations have been or arebeing conducted in a gassy coal seam. The primary need under this lattercondition is to reduce the gas hazard involved in the mining itself. Bythe installations and procedure of my invention this primary need is metin a manner which also permits the collection and delivery of themaximum amount of the highest quality gas from the coal still remainingin the workings when further removal of coal becomes uneconomical.

I have illustrated and described two interconnected sealed chambersystems of this kind, one of which operates under a mean pressureslightly below that of the active mine, and the other of which operatesunder a mean pressure slightly above that of the active mine. In bothcases direct communication with the atmosphere is provided throughspecial venting appliances, and in both cases the pumps used for thedelivery of this gas to a point of consumption are also employed toreduce the pressure build-up within the sealed chambers. For simplicitythe instrumentation required for these systems has been reduced todiagrammatic showings and not duplicated.

For th purpose of making m invention clear, I have shown and describedin some detail and by the aid of specific examples the principles andmethod of use of my invention. Through use of these principles andmethods of application, it is possible to add material amounts ofnatural gas to the Worlds current fuel supplies, while at the same timeeliminating or reducing the hazards of fire and explosion associatedwith the active mining of coal in gassy underground mines. The sameinstallations and methods may be used in mines in which active mininghas been temporarily or indefinitely suspended since these installationsbest lend themselves to the state of the mine when active operationswere suspended and leave open the possibility of re-opening the mine atminimum expense. Often it is possible by transfer of gas from oneunderground chamber to another, as illustrated in the example given, tosave the cost of gathering and transmission lines and at the same timesecure greatest uniformity of gas quality.

I do not regard the invention as limited to any of the details shown ordescribed save in so far as such limitations are included within theappended claims in which it is my intention to claim all noveltyinherent in my invention as broadly as is permissible in view of theprior art.

What I claim is:

1. The method of minimizing gas hazards in underground coal miningoperations and simultaneously making possible the recovery of themaximum amount of highest quality gas at a uniform rate declining onlywith the slow decline of the rate of gas evolution, which methodcomprises sealing ofif a mined-out section of the underground workingsby a partition wall across the entry connecting it with the areas inwhich active mining continues, connecting the sealed chamber with thesurface by a gas flow conduit normally closed against the atmosphere,forcibly withdrawing gas from the chamber at a rate not exceeding thatat which it is evolved from the exposed coal surfaces under a constantpressure approximating atmospheric pressure, and opening such chamber tothe atmosphere through said flow conduit only when accumulation ofexcess gas or a falling barometer causes the chamber pressure to exceedatmospheric pressure by a predetermined small amount.

2. An operation conducted according to preceding claim 1, and includingthe additional steps of employing a second partition spaced away fromsaid first named partition to form a gas-lock, and forcibly circulatinga portion of the withdrawn gas through said gas-lock while maintainingtherein a pressure varying with the barometer and always equal to thepressure in the adjacent active mine area, whereby leakage of gas fromthe sealed chamber into the active mine or leakage of air from theactive mine into the sealed chamher under pressure differentials createdby forcible withdrawal of gas or by changing barometric pressures areboth prevented.

3. An operation conducted according to preceding claim 1 involving aplurality of scaled chambers, and including the additional steps offorcibly transferring gas withdrawn from one sealed chamber into anothersealed chamber and forcibly withdrawing the transferred gas as well asthe evolved gas from the second chamber.

4. An operation conducted according to preceding claim 1 involving aplurality of sealed chambers and involving the additional steps offorcibly transferring gas withdrawn from one sealed chamber into anothersealed chamber at a point adjacent the sealing partition and forciblywithdrawing the transferred gas as well as the evolved gas from thesecond chamber at a point remote from the sealing partition.

5. An operation conducted according to preceding claim 1 in which thegas is forcibly removed from the sealed chamber at a point remote fromthe sealing partition.

FRANK A. HOWARD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,358,920 Garrison Sept. 26, 1944FOREIGN PATENTS Number Country Date 6,640 Great Britain of 1894

